The miniaturization of electronic devices has created a demand for very small but powerful electrochemical cells. Cells that utilize an alkaline electrolyte are known to provide high energy density per unit volume, and so are well suited for applications in miniature electronic devices such as hearing aids, watches and calculators. However, alkaline electrolytes, such as aqueous potassium hydroxide and sodium hydroxide solutions, have an affinity for wetting metal surfaces and are known to creep through the sealed metal interface of an electrochemical cell. Leakage in this manner can deplete the electrolyte solution from the cell and can also cause a corrosive deposit on the surface of the cell that detracts from the cell's appearance and marketability. These corrosive salts may also damage the device in which the cell is housed. Typical cell systems where this problem is encountered include silver oxide-zinc cells, nickel-cadmium cells, air depolarized cells, and alkaline manganese dioxide cells.
In the prior art it has been a conventional practice to incorporate insulating gaskets between the cell container and cover so as to provide a seal for the cell. Generally, the gasket must be made of a material inert to the electrolyte contained in the cell and the cell environment. In addition, it had to be flexible and resistant to cold flow under pressure of the seal and maintain these characteristics so as to insure a proper seal during long periods of storage. Materials such as nylon, polypropylene, ethylene-tetrafluoroethylene copolymer and high density polyethylene have been found to be suitable as gasket materials for most applications.
In conventional thin miniature cell constructions, a separator is disposed over the cathode to isolate and protect the cathode from contacting the anode and the cover terminal of the cell. If the edge of the cathode cuts the separator as can easily happen in thin miniature cells of about 0.060 inch height or less, the exposed edge of the cathode may contact the cover terminal of the cell thereby causing an internal short. Alternatively, in miniature silver oxide cells, silver migration may occur around the separator and thus also cause internal shorting. As stated above, this problem of internal shorting is particularly troublesome in a thin miniature cell of about 0.060 inch height or less since the upper edge of the cell container has to be turned over onto the gasket a sufficient amount to effectively seal the cell while also insuring that there will be sufficient terminal clearance for the cell. Terminal clearance, as used herein, refers to the distance that a cell cover extends above the peripheral sealing edge or shoulder (gasket and/or edge of the container), of the cell. Generally, a thin electrochemical cell fits snugly into a cavity in an electronic device in which it is to be used whereby the cell container contacts the positive lead of the device and the cell cover contacts the negative lead of the device. If the sealing gasket and/or edge of the container protrudes above the cell cover, then this protrusion may prevent the cell from properly contacting the negative lead of the device in which it is housed. If the cell cover is eclipsed by the container sidewall, then the sidewall may contact the negative lead of the electronic device, shorting out the circuit.
Many attempts have been made to refine the sealing gasket that is disposed between the cell container and cell cover of thin miniature cells. These approaches have primarily addressed the problem of electrolyte leakage and/or insufficient terminal clearance for proper electrical contact of thin miniature cells when positioned in a battery-powered device.
It is an object of the present invention to provide a means for adequately isolating and protecting the cathode of thin miniature cells from contacting the anode and cover terminal of the cell.
Another object of the present invention is to provide an improved means to effectively prevent internal shorting of the cell's cathode to the anode and cover terminal of thin miniature cells, such as silver oxide miniature cells.
Another object of the present invention is to provide a method for assembling thin miniature cells that is suited for automatic production while insuring the proper isolation of the cathode from the anode and cover terminal of the cell to effectively prevent internal shorting.
The foregoing and additional objects will become more fully apparent from the following description.